Conventional gas combustors burn gaseous fuel combined with compressed air in an approximately stoichiometric mixture. Burning a stoichiometric fuel and air mixture in a diffusion flame produces high combustion zone temperatures, which promote formation of oxides of nitrogen ("NOx"), an atmospheric pollutant. It is well known that pre-mixing fuel and air in lean conditions, typically in which the fuel to air ratio is less than approximately 0.035 by weight, produces combustion products with a low NOx concentration.
Researchers have devoted considerable effort toward developing a low-NOx gas combustor that burns fuel and air in lean conditions after pre-mixing. U.S. Pat. No. 5,361,586 to McWirter et al., incorporated by reference herein in its entirety, describes a multi-annular combustor having a plurality of annular passages concentrically disposed around a gas pilot apparatus. Each of the annular passages may be linked together or may have a separately controlled fuel valve for controlling the fuel flow rate and fuel to air ratio within each annular passage.
A conventional gas pilot, like the one at the center of the multi-annular combustor, comprises a main fuel tube enclosed concentrically by a supplemental fuel tube. Fuel flowing from the end of the main fuel tube burns upon contact with air from the surrounding supplemental fuel tube in a diffusion type flame. Unfortunately, the conventional pilot produces unstable flame conditions at many of the fuel to air ratios and flow rates commonly required by multi-annular combustors such as that of the '586 patent. Unstable flame conditions include flameout, flashback, and high dynamic pressure indicating noise and vibration.
Combustors with a gas pilot are used frequently to produce hot gases to drive a combustion turbine. Accelerating and operating a combustion turbine requires pilot flame stability over a wide range of combustor operating conditions. The wide range of fuel and air throughputs, fuel to air ratios, amounts of premixing, and ambient temperature conditions exacerbate flame instability problems of a conventional pilot.
There exists a need for an effective method of establishing a stable gas pilot flame in combustors over a wide range of operating conditions.